This invention relates to improvements in an exhaust gas purifying system for purifying exhaust gas or burnt gas discharged from a combustion device such as an internal combustion engine, a burner, a furnace or the like, and particularly to the improvements in the exhaust gas purifying system using a catalyst for reducing nitrogen oxides (NOx) in exhaust gas discharged during a lean-burn operation in which an air-fuel mixture leaner than a stoichiometric level is supplied to the combustion device, under effective use of hydrogen.
It is well known that a three-way catalyst was used to oxidize carbon monoxide (CO) and hydrocarbons (HC) and to reduce nitrogen oxides (NOx) contained in exhaust gas discharged from an internal combustion engine of an automotive vehicle or the like. The three-way catalyst can effectively function in a stoichiometric atmosphere (exhaust gas having a stoichiometric air-fuel ratio). However, such three-way catalyst cannot function to reduce nitrogen oxides in a lean atmosphere (exhaust gas having a leaner air-fuel ratio than the stoichiometric level). In this regard, a technique for reducing nitrogen oxides in the lean atmosphere has been proposed in Japanese Patent No. 2600492 in which a NOx trap agent can trap NOx in the lean atmosphere and can release NOx by enriching air-fuel ratio of exhaust gas flowing to the NOx trap agent so as to reduce the released NOx.
However, in such techniques using the three-way catalyst or disclosed in Japanese Patent No. 2600492, it is required to periodically or intermittently supply HC and CO as a reducing agent (reducing component or gas) to promote reduction reaction of NOx. In this connection, HC and CO which has not been consumed in the NOx reduction reaction is required to be oxidized. This will be accomplished by a measure of oxidizing HC and CO by making oxidation reaction simultaneously on the NOx treating catalyst, or another measure of oxidizing excessive HC and CO in a three-way catalyst or the like disposed downstream of the NOx treating catalyst.
However, such a NOx treating catalyst or a three-way catalyst disposed downstream of the NOx treating catalyst considerably far from the engine, and therefore the temperature of exhaust gas flown to the three-way catalyst is unavoidably lowered, thereby making it impossible to exhibit a sufficient oxidizing performance for HC and CO. Particularly immediately after engine starting, sufficient oxidation of HC and CO is difficult to be made.
Additionally, when HC and CO components in exhaust gas is increased to lower oxygen concentration in exhaust gas and to reduce NOx as discussed above, it is impossible to make a vehicle running under the lean-burn operation, thereby exhibiting an insufficient fuel economy improvement effect.
It is, therefore, an object of the present invention is to provide an improved exhaust gas purifying system which can effectively overcome drawbacks encountered in conventional techniques for reducing NOx in exhaust gas.
Another object of the present invention is to provide an improved exhaust gas purifying system which can effectively remove NOx, HC and CO from exhaust gas while obtaining a high fuel economy improvement effect.
A further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can remove NOx in exhaust gas at a high efficiency throughout a time immediately after engine starting and a time of making steady state engine operation and effectively remove HC and CO particularly during a low temperature engine operation immediately after engine starting, while obtaining a high fuel economy improvement effect upon making a lean-burn operation of the engine.
An aspect of the present invention resides in an exhaust gas purifying system comprising a NOx treating catalyst for reducing NOx disposed in an exhaust gas passageway of a combustion device, to reduce NOx in presence of reducing components in exhaust gas. Additionally, a hydrogen enriching device is disposed upstream of the NOx treating catalyst with respect to flow of exhaust gas from the combustion device and arranged to increase a ratio of hydrogen to total reducing components in at least one of combustion gas and exhaust gas so as to meet relations represented by following formulae (1) and (2), when reduction of NOx is carried out by the NOx treating catalyst:
[H2/TR]d greater than [H2/TR]uxe2x80x83xe2x80x83(1) 
[H2/TR]dxe2x89xa70.3xe2x80x83xe2x80x83(2) 
where [H2/TR]u is a ratio between a concentration [H2]u of hydrogen and a concentration [TR]u of total reducing components in at least one of exhaust gas in the exhaust gas passageway upstream of the hydrogen enriching device and combustion gas in a state before undergoing the hydrogen ratio increasing by the hydrogen enriching means; and [H2/TR]d is a ratio between a concentration [H2]d of hydrogen and a concentration [TR]d of total reducing components in exhaust gas in the exhaust gas passageway upstream of the NOx treating catalyst and downstream of the hydrogen enriching device.
Another aspect of the present invention resides in an exhaust gas purifying system of a multiple step control type in combination with an internal combustion engine having an exhaust gas passageway. The engine includes a combustion system having a combustion control device for controlling at least one selected from the group consisting of operating parameters of the engine and combinations of the operating parameters, the operating parameters including fuel injection timing, spark timing, opening and closing timings of intake and exhaust valves of the engine. The exhaust gas purifying system includes a NOx treating catalyst for reducing NOx disposed in the exhaust gas passageway to reduce NOx in presence of reducing components in exhaust gas. A hydrogen enriching device is disposed upstream of the NOx treating catalyst with respect to flow of exhaust gas and includes at least one selected from the group consisting of a hydrogen producing catalyst containing at least one noble metal, a CO and HC selective oxidation catalyst containing zirconium oxide, a catalyst containing solid acidic zirconium oxide, and a device for supplying hydrogen-contained gas produced by using hydrocarbon fuel and air, from outside of the exhaust passageway. The hydrogen-contained gas supplying device includes at least one of a first hydrogen-contained gas supplying device having a hydrogen-contained gas producing catalyst for promoting reaction for producing hydrogen-contained gas from the hydrocarbon fuel, and a device for supplying the hydrocarbon fuel and air to the catalyst, and a second hydrogen-contained gas supplying device for producing hydrogen-contained gas by using hydrocarbon fuel and exhaust gas under heat. The hydrogen enriching device is arranged to increase a ratio of hydrogen to total reducing components in at least one of combustion gas and exhaust gas so as to relations represented by the following formulae (1) and (2), when reduction of NOx is carried out by the NOx treating catalyst:
[H2/TR]d greater than [H2/TR]uxe2x80x83xe2x80x83(1) 
[H2/TR]dxe2x89xa70.3xe2x80x83xe2x80x83(2) 
where [H2/TR]u is a ratio between a concentration [H2]u of hydrogen and a concentration [TR]u of total reducing components in at least one of exhaust gas in the exhaust gas passageway upstream of the hydrogen enriching device and combustion gas in a state before undergoing the hydrogen ratio increasing by the hydrogen enriching means; and [H2/TR]d is a ratio between a concentration [H2]d of hydrogen and a concentration [TR]d of total reducing components in exhaust gas in the exhaust gas passageway upstream of the NOx treating catalyst and downstream of the hydrogen enriching device.
A third aspect of the present invention resides in a method of purifying exhaust gas from a combustion device provided with an exhaust gas purifying system including a NOx treating disposed in an exhaust gas passageway of the combustion device, the NOx treating catalyst reducing NOx in presence of reducing components in exhaust gas. The method comprises increasing a ratio of hydrogen to total reducing components in at least one of combustion gas and exhaust gas to be supplied to the NOx treating catalyst so as to meet relations represented by the following formulae (1) and (2), when reduction of NOx is carried out by the NOx treating catalyst:
[H2/TR]d greater than [H2/TR]uxe2x80x83xe2x80x83(1) 
[H2/TR]dxe2x89xa70.3xe2x80x83xe2x80x83(2) 
where [H2/TR]u is a ratio between a concentration [H2]u of hydrogen and a concentration [TR]u of total reducing components in at least one of exhaust gas in the exhaust gas passageway upstream of the hydrogen enriching device and combustion gas in a state before undergoing the hydrogen ratio increasing; and [H2/TR]d is a ratio between a concentration [H2]d of hydrogen and a concentration [TR]d of total reducing components in exhaust gas in the exhaust gas passageway upstream of the NOx treating catalyst and in a state after undergoing the hydrogen ratio increasing.